July 12, 2003 Update

 

Catalyst Bale

 

We tried an alternate catalyst design fro Catalytic Products International this week.  It is basically a mass of thin stainless steel ribbons that have been crimped like heating elements, then coated with a catalyst.  It is provided in a pack wired together like a hay bale.  We were hoping that a platinum based catalyst on a stainless steel base would be a metallic bond, but it turned out to be more of a platinum-black powder coating, and the adhesion was terrible.  It was probably fine for gas processing (the intended application), but you could rub the coating off with gloved fingers, and rocket engine liquid / high speed gas flows took it right off.  Baking it on under flame helped a bit, but not nearly enough.  When we fired an engine with this catalyst in it, we wound up with a big ring of black goo on the ground under the nozzle where all the catalyst had been washed off.

 

http://media.armadilloaerospace.com/2003_07_12/catalystBale.jpg

 

We have one more form of metallic catalyst from CPI to test next week, but if is the same type of adhesion, it isn’t going to work for us either.  It is pretty clear that we need a metallic bond, so we will probably have to get some material electroplated.

 

Forced Preheat

 

The plumbed-in preheating that we have used in our last couple tests used a large, high-flow propane torch assembly with a tube on the end to meter propane and air through the top of the engine.  Because it relies on venturi suction for air supply, we can’t force the gas mixture through any pressure drop, like a check valve, and the mixture ratio can’t be adjusted independent of the total flow.  We have done the tests with a manual ball valve at the heater port on the engine, but if we get any of these preheated packs working well enough to go in a vehicle with four engines, we want to be able to make a connection in a single place, and have all the engines heated from there, which will require check valves to isolate the engines (or four manual valves, but we don’t like that solution).  We need to move to a forced-air plumbing arrangement.

 

We got two new compressed gas cylinders in this week, a hydrogen cylinder, and a breathing air cylinder.  We don’t want to use pure oxygen for the preheat, because the flame temperatures would be far too high.   The extra nitrogen in compressed air is a good buffer.

 

The hydrogen was interesting – if you just blow hydrogen gas over a platinum catalyst in an open-air environment, it starts heating it up even if it is at room temperature.  If the catalyst has been preheated at all with a torch, blowing hydrogen onto it causes spontaneous ignition.  This differs from propane over the catalyst quite a bit – propane won’t do anything on a cold catalyst, but if it is heated nearly red hot with a torch, propane flow will keep it red hot, but won’t actually start a flame.

 

There is a good NACA document on hydrogen burning properties at:

 

http://naca.larc.nasa.gov/reports/1958/naca-report-1383/naca-report-1383.pdf

 

The fact that hydrogen flames start so spontaneously, and over such a wide mixture ratio, caused us some problems.  When we flowed propane / air mixtures in from the top of the engine, it stayed a cool gas mixture in the plumbing, and only burned on the active catalyst surfaces that had been heated.  When we flowed hydrogen / air mixtures, it would burst into flame inside our mixing system above the engine, which we would prefer to keep cool.  Only when the mixture ratio was extremely lean would it only burn at the catalyst, and at that point the resulting temperature was too low to heat the catalyst the way we wanted.  Hydrogen can actually make a quite cool flame, down to 1200 K / 1700 F or so.

 

We weren’t getting the catalyst hot enough with a non-burning flame, so we cranked up the hydrogen for a while, which made the top of the engine very hot.  When we saw some little glowing bits fall out of the engine nozzle, we realized we had probably gone too far.  After a failed engine test, we opened it up and found that our stainless steel spreading plate had been completely slagged, along with a fair amount of catalyst beneath it.

 

http://media.armadilloaerospace.com/2003_07_12/slagged.jpg

 

We moved back to the more controllable propane mixtures for the rest of our tests, and we also moved over to using an air compressor instead of the bottled air, which was being consumed very rapidly.  This arrangement works fine, but it does require torch heating the bottom of the catalyst, and it takes a while for the heat to propagate up the catalyst so that it starts catalytic burning through the entire thing.  I need to buy a couple gas flow meters for this type of work.  Right now, we can tell if the gas mixture is fuel rich by lighting the gas coming out of the nozzle, but that is about it.

 

Big Vehicle Work

 

We are building a conformal box for the final parachute packing that fits up against the tank bottom.  The sides are cut from thin Nomex honeycomb boards, while the bottom will be a hand lay-up of fiberglass cloth formed to the tank contour.  We will be building a custom packing box with a contour foamed bottom that the box can be placed in for vacuum / pressure packing of the canopy.

 

http://media.armadilloaerospace.com/2003_07_12/chuteBox.jpg

 

We installed a second large chain hoist on the longer ceiling girder at our shop to make moving the big vehicle around easier, and we are putting big castors on the vehicle cradle that can properly handle the fully loaded weight.

 

The big tank liner is cross-linked polyethylene, which is compatible with peroxide, but the big manway flange closure is a fiber reinforced vinyl-ester composite, which is not peroxide compatible.  The most mass efficient solution would be to replace the 2” thick manway closure with a dished aluminum flange, but we have chosen to go a simpler route and just add an aluminum plate between the tank and the closure, so the existing closure still takes all the pressure loads, while the aluminum plate provides peroxide compatibility.  We are keeping the central post as the main parachute / lifting attachment point, and adding two 2” pipe ports through the closure for plumbing to the engines and our fountain-fill port.  Drilling through the thick composite plate wasn’t as bad as I feared, but we did use up a new bimetal hole saw in the process.

 

http://media.armadilloaerospace.com/2003_07_12/flangePlate.jpg